Pressure sensors for implantable applications have been developed in the past. A focus of the pressure sensors in the art has been to monitor, for example, intracranial pressures, intrapleural and blood pressure. Recently, particular attention has been drawn to monitor and treat a condition known as hydrocephalus. Hydrocephalus, also known as “water in the brain,” is a medical condition in which there is an abnormal accumulation of cerebrospinal fluid (CSF) in the ventricles, or cavities, of the brain. This may cause increased intracranial pressure inside the skull and progressive enlargement of the head, convulsion, tunnel vision, and mental disability. Hydrocephalus can also cause death.
Pressure sensors exist today that can be implanted inside the cranium for a short period of time up to a maximum of a few weeks, with connecting wires or conduits passing through a wound in the scalp. These devices for example, are used in treating acute conditions such as traumatic brain injury (TBI) or monitoring and diagnosing chronic conditions such as hydrocephalus. However, pressure sensors must have certain attributes in order to be effective in long term (greater than four weeks) pressure monitoring applications and because today no devices are commercially available that have all the necessary attributes, hydrocephalus and TBI are treated without the benefit of continuous, long term intracranial pressure information. Accordingly, the shortcomings of current devices creates a severe limitation in monitoring the effectiveness of ventriculoperitoneal shunts, which are commonly used in treating hydrocephalus and in treating acute brain injury. In such instances an additional concern is the replacement of short term sensors that have failed with the attendant risk of infection accompanying sensor use.
The following attributes are required for a pressure sensor in order for it to be suitable for long term implantation and monitoring applications in the brain. The pressure sensors must be diminutive in size for ease of implant as well as to minimize disturbance to the tissue environment surrounding the pressure sensor and they must use wireless communication so that no wires, conduits, or other components require a passageway through the skin. They must be formed of break-resistant, non-toxic and everlasting bio-compatible materials to minimize the patient's immune reaction to the introduction of a foreign body and to prevent tissue injury from corrosion byproducts. The implanted pressure sensor must provide pressure values that remain reliable so that treatment decisions may be made with confidence for the lifetime of the sensor to preclude sensor explant for adjustment purposes. Accordingly, the sensor must be constructed in a manner so that its measurement accuracy remains within a prescribed tolerance range independent of physical movements, temperature changes or other environmental influences it may experience. Additionally, since physiologic parameters such as internal pressures are measured with reference to local atmospheric pressure, a system utilizing an implanted pressure sensor must include provision to account for the atmospheric pressure around the patient. In such case, the use of an external controller, for example, that includes a reliable and accurate pressure sensing device is required.
There have been attempts in the past to address the issue of internal pressure measurement. For example U.S. Pat. No. 4,846,191 to Brockway et al. describes measurement of physiologic pressure by placement of a pressure transmitting catheter within a blood vessel or other structure within which pressure is to be measured. Aspects of the device include using a flexible catheter for transmitting pressure measurements from an implanted site to a pressure sensor located a distance away from the site. Typically, the pressure sensor is embedded in the scalp or just under the skin. The catheter is elongated, filled with a low viscosity fluid and is plugged with a gel. Since the catheter is compliant, the transmission of accurate and reliable pressure values is undependable. The application of fluid filled elongated catheters used in measuring pressure signals from a lateral ventricle has also been described in U.S. Pat. No. 4,519,401 to Ko and Leung. The system described in Ko also suffers from the same disadvantages as those devices in the art using fluid filled catheters in that the accuracy and reliability of pressure measurements remain undependable. Another example is U.S. Pat. No. 3,697,917 to Orth et al. that discloses a planar silicon diaphragm, anodically bonded to a cylindrical glass support and mounted in a metal tube by means of an O-ring. Deflections of the diaphragm are measured by piezoresistive strain gauges that have metal wire conductors that extend beyond the sensor housing and into the environment surrounding the housing. The device of Orth, is severely limited in its application since it is not implantable because the housing is not sealed and cannot be made hermetic due to the use and orientation of the O-ring. Yet another example is found in U.S. Pat. No. 3,958,558 to Dunphy et al. which describes a pressure transducer that includes a coaxially variable capacitor or coaxially variable inductor in alternate arrangements, wherein a bellows is mechanically coupled to the variable component to vary the value of the component in response to pressure changes of the fluid in which the bellows is immersed. Varying of the component value by the bellows, causes a change in resonant frequency of an L-C circuit which is sensed by an external source of variable frequency oscillatory energy which in turn is indicative of the level of fluid pressure being sensed. The long term reliability and accuracy of the pressure measurements of the disclosed transducer remains a question, at least, because of the involved mechanical arrangement of the bellows and the coaxially variable components will experience hysteresis and materials fatigue causing calibration drift.